A complex electrical system is oftentimes designed to include a plurality of modular circuit packs. These circuit packs, which individually perform well-defined integral functions for the system, work synergistically to realize various system capabilities. This design approach allows a system to be flexibly configured by the selection of various combinations of modular circuit packs.
Modular circuit packs in a system are typically assembled in a frame, specifically, in plug-in slots which are defined by circuit pack guides. The circuit packs are connected to one or more buses which are wired on a backplane of the frame and via which electrical signals are intercommunicated between the circuit packs. To this end, a connector on a circuit pack mates mechanically and electrically with a corresponding backplane connector at the end of a plug-in slot. This backplane connector, depending on the number of contacts it has, may be connected to one or more of the buses on the backplane.
The above-described design approach is most advantageous for a simple electrical system that requires only one bus. It is not so, as explained hereinbelow, for a complex electrical system which normally requires a plurality of buses to accommodate different schemes for communications between individual circuit packs. These schemes may be, for example, a time-division-multiplexed scheme, a data packetized scheme, etc., wherein each scheme is associated with a different bus configuration. Each bus configuration specifies the functions of the individual leads of a bus, and restricts, for example, the signal waveforms from a circuit pack to drive the individual bus leads.
The foregoing variety of bus configurations unduly limits system expansion. This limitation arises because it is normally uneconomical to have the backplane connector of each plug-in slot connected to all of the differently configured buses within the system since only a subset of these buses is used by any circuit pack. Indeed, it is more economical to have different types of plug-in slots where each type of plug-in slot is wired to a particular subset of the different buses and these subsets are chosen to be usable by one or more of the circuit packs. Because the plug-in slots of different types are not interchangeable, one must reserve additional slots to anticipate system expansion. However, due to the uncertainty inherent in the forecast of future circuit pack usage, the determination of the number reserved for each type of slot is, at best, speculative. It is very likely that new circuit packs which are subsequently added for system expansion may exhaust the reserved slots of a particular type while many of the other types remain unused. The further need of circuit packs that require that particular type of slot often causes a system user to wastefully replace the whole system. More significantly, as technology evolves, it is oftentimes desirable to enhance the capability of a system in use by the addition of circuit packs requiring a bus configuration not originally contemplated. As a consequence thereof, the system user who desires the enhanced capability is faced with no choice but to, again, wastefully replace the current system with one of a newer model, which will probably be replaced in the near future for the same above-stated limitation.
Accordingly, it is desirable to have a bus arrangement capable of supporting a plurality of bus configurations, which does not require accurate forecast of future circuit pack usage and which can readily accommodate one or more of bus configurations not originally contemplated.